1. Field of the Invention
The present invention relates to heat pumps useful for hot water heaters, air-conditioners, and the like, and more particularly to a heat pump furnished with a mechanism for recovering energy by an expander.
2. Description of the Related Art
A heat pump employing an expander in place of an expansion valve can recover the expansion energy of refrigerant as electric power or mechanical power. As the expander, in many cases a positive displacement expander is used that has a space with a variable capacity for introducing and expanding refrigerant therein. The energy recovery with the expander has a significant value, particularly in the transcritical cycle of carbon dioxide in which the high-pressure side reaches a supercritical state of the refrigerant.
Because of its structure, the expander cannot recover energy unless the refrigerant passes through it in a predetermined direction. In a heat pump used for an air-conditioner, however, it is basically required that the refrigerant should flow in opposite directions when in a cooling operation and when in a heating operation because it is necessary to use a heat exchanger installed indoors as a radiator during the heating operation but as an evaporator during the cooling operation.
JP 2001-66006A discloses a heat pump capable of energy recovery with an expander in both cooling and heating operations. This heat pump is designed so that the refrigerant can flow through the expander in the same direction in both operations of cooling and heating by switching a four-way valve. Furthermore, in this heat pump, the expander and a compressor are connected to the same rotating shaft. In other words, they are directly coupled, in order to use the energy recovered by the expander directly for operating the compressor.
In the heat pump in which the expander and the compressor are directly coupled, the expander and the compressor operate at the same rotational speed and therefore it is impossible to vary the ratio between the displacement of the expander and the displacement of the compressor according to the operation condition. For that reason, the heat pump of this type has difficulty in performing a smooth operation according to the operation condition, although it has good efficiency in energy recovery. JP 2003-121018A discloses a heat pump that decreases this difficulty.
As illustrated in FIG. 20, JP 2003-121018A discloses a heat pump in which two four-way valves 131 and 134 are disposed in pipes 110 so that the refrigerant can flow in the same direction through an expander 104 and a compressor 101 in both operations of cooling and heating by switching the four-way valves 131 and 134, as in JP 2001-66006A. In an air-conditioner employing this heat pump, the passages shown by solid lines in the four-way valves 131 and 134 are selected during heating so that an indoor heat exchanger 132 functions as a radiator and an outdoor heat exchanger 136 functions as an evaporator. In this air-conditioner, the passages shown by broken lines in the four-way valves 131 and 134 are selected during cooling so that the indoor heat exchanger 132 functions as an evaporator and the outdoor heat exchanger 136 functions as a radiator. In this heat pump, the expander 104 and the compressor 101 are coupled directly to share a single rotating shaft, and this rotating shaft is driven by a motor 130.
In the heat pump disclosed in JP 2003-121018A, an expansion valve (bypass valve) 139 is disposed in a bypass circuit 120 disposed in parallel with the expander 104, and an expansion valve 105 is disposed in series with the expander 104. The opening of the expansion valve 105 or the expansion valve 139 is controlled according to the operation condition.
As discussed above, although the heat pump in which the expander and the compressor are directly coupled is advantageous in energy recovery, it cannot change the displacement ratio between the expander and the compressor according to an operation condition. For example, if the expander is designed based on a standard condition in a cooling operation, the displacement of the expander will be too large in a heating operation with respect to the required value. For that reason, in the heat pump disclosed in JP 2003-121018A, the bypass valve 139 is fully closed during a heating operation, and the opening of the expansion valve 105 is controlled as appropriate. If the opening of the expansion valve 105 is reduced, the specific volume of the refrigerant flowing into the expander 104 will increase. In a cooling operation, the displacement of the expander 104 may become less than the required value. When this is the case, the expansion valve 105 is fully opened, and the opening of the bypass valve 139 is controlled as appropriate. Thus, the heat pump disclosed in JP 2003-121018A is capable of smooth cycle operations according to operation conditions.
FIG. 21 is a Mollier diagram illustrating the refrigeration cycle of the heat pump shown in FIG. 20. The refrigerant that is discharged from the compressor 101 and that is in the state a at a high pressure PH radiates heat at the indoor heat exchanger 132 or the outdoor heat exchanger 136 that functions as the radiator 104, and then reaches state b. The refrigerant undergoes isentropic expansion in the expander 104, reaching state c at an intermediate pressure PM, and then further undergoes isenthalpic expansion at the expansion valve 105, reaching a state d at a low pressure PL. The refrigerant then absorbs heat at the outdoor heat exchanger 136 or the indoor heat exchanger 132 that functions as the evaporator, reaching state e, and thereafter flows into the compressor 101. In this heat pump, the energy corresponding to an enthalpy difference W2 between state b and state d is recovered by the expander 104. Therefore, it is sufficient that, basically, the mechanical power corresponding to a value (W1-W2), obtained by subtracting the enthalpy difference W2 from a enthalpy difference W1 between state a and state e, is input to this heat pump.
JP 2003-121018A also discloses a heat pump in which, as illustrated in FIG. 22, the expansion valve 105 is disposed on the upstream side of the expander 104. This heat pump has the same configuration as that of the heat pump shown in FIG. 20 except for the positions of the expansion valve 105 and a receiver 100 for the refrigerant. FIG. 23 shows a Mollier diagram illustrating the refrigeration cycle in the heat pump shown in FIG. 22. This refrigeration cycle is the same as the refrigeration cycle shown in FIG. 21 except that the isenthalpic expansion in the expansion valve 105 (the expansion from state b to state c in FIG. 22) is performed prior to the isentropic expansion in the expander 104 (the expansion from state c to state d in FIG. 23).
In the heat pump disclosed in JP 2003-121018A, the specific volume of the refrigerant flowing into the expander 104, in other words, the pressure of the refrigerant flowing into the expander 104, is controlled by adjusting the opening of the expansion valve 105 disposed on the upstream side or downstream side of the expander 104.
However, when the opening of the expansion valve 105 is controlled in order to control the pressure PM of the refrigerant flowing into the expander 104, the refrigeration cycle as a whole will shift toward the high-pressure side or the low-pressure side, and as a result, the pressure PH of the high-pressure side of the refrigeration cycle changes. Even if the pressure PM can be controlled in the refrigeration cycle, it will be difficult to keep the efficiency of the heat pump high as long as that controlling is accompanied by an unintended change in the pressure PH of the high-pressure side.
Thus, the control mechanism of the heat pump disclosed in JP 2003-121018A has a problem that the pressure PM of the refrigerant flowing into the expander 104 and the pressure PH of the high-pressure side of the refrigeration cycle cannot be controlled independently. One of the reasons is that one of the expansion valves 105 and 139 is fully opened or fully closed and only the other one is controlled; also, an additional factor that makes it difficult to resolve the problem is that, in the heat pump, the two expansion valves are not disposed in a manner that makes it easy to control both the pressure PM and the pressure PH.
As illustrated in FIGS. 20 and 22, the receiver 100 is in many cases installed in a heat pump that is operated under conditions that require considerably different amounts of refrigerant, such as in a cooling operation and in a heating operation, in order to adjust the amount of refrigerant that circulates in the heat pump. The receiver 100 prevents refrigerant from flowing into the expander 104 in an excessive amount by temporarily reserving the refrigerant.
However, when the reliability of the apparatus is ensured by the receiver, the size of the heat pump increases, and the amount of refrigerant to be charged therein becomes large. The size increase of the heat pump limits the installation position and does not meet the demands of the user. Reducing the amount of refrigerant to be charged has also been a social demand from the viewpoint of reducing environmental load.
The two problems discussed above—the first problem that the pressure PM of the refrigerant flowing into the expander and the pressure PH of the refrigerant in the high-pressure side of the refrigeration cycle cannot be controlled independently, and the second problem that the reliability of the apparatus needs to be ensured by the receiver—become evident in the heat pump in which the expander and the compressor are directly coupled, as illustrated in FIGS. 20 and 22, but these problems also exist in the heat pump in which the expander and the compressor are not directly coupled.
For example, by connecting the expander to a power generator, it is possible to construct a heat pump that can recover the energy originating from the expansion of refrigerant as electric power, and in this case, it is not necessary to couple the expander and the compressor directly. Nevertheless, with the heat pump of this type as well, it is desirable to control both the pressure PM of the refrigerant flowing into the expander and the pressure PH of the refrigerant in the high-pressure side of the refrigeration cycle to be desired values, in order to achieve a smooth cycle operation according to operation conditions. Moreover, in the heat pump of this type as well, a receiver is usually installed in order to prevent refrigerant from flowing into the expander 104 in an excessive amount.